In order to mitigate the noise generated by a jet engine, it is known to form the jet engine inlet and exhaust duct walls of noise absorbing material. This is particularly desirable for commercial passenger aircraft, since such aircraft are required to meet stringent government noise regulations. Such noise suppression for commercial passenger aircraft is also desirable so as to enhance the comfort of the passengers thereof.
Additionally, many localities have noise regulations which limit the amount of noise that an aircraft may make in order to prevent annoying nearby residents. This sometimes limits the types of aircraft that may utilize a particular airport. It also frequently imposes procedural restrictions upon aircraft which would otherwise be undesirable. For example, aircraft may be required to fly over designated neighborhoods at a prescribed minimum altitude. Flying at such a minimum altitude may undesirably increase fuel consumption and also possibly accelerate engine wear (particularly when the minimum altitude must be reached immediately after takeoff).
According to contemporary methodology, a noise suppressing material sold under the name DYNAROHR (a registered trademark of Rohr Industries, Inc.) is utilized extensively of this purpose. The DYNAROHR product is formed of a honeycomb material which is disclosed in U.S. Pat. No. 4,379,191, the contents of which are hereby incorporated by reference. This honeycomb material comprises a core having a plurality of open cells. The core is sandwiched between an outer non-porous layer and an inner porous layer. The inner porous layer is in fluid communication with the open cells of the honeycomb material. A microporous sheet material, such as one comprised of finely woven stainless steel cloth, is bonded over the porous sheet and forms a part of the inner surface of the jet engine's inlet duct.
As those skilled in the art will appreciate, at least a portion of the acoustic energy incident on such noise suppression material is not absorbed but is re-radiated elsewhere. The intensity of the re-radiated acoustic energy provides a measure of the absorption or effectiveness of the noise suppressing material.
Various devices and methodologies are well known for measuring the acoustic absorption properties of materials. For example, one such methodology comprises disposing the sound absorbent material in an anechoic chamber proximate a sound source and a sound sensor, and then making sound measurements both with the material in place and removed, such that a comparison of the sound measurements may be made. However, as those skilled in the art will appreciate, such contemporary methodology requires the use of an anechoic chamber, which is inconvenient, expensive, not easily transported, and incapable of utilizing the high sound levels generated in the intake of a commercial jet engine.
As such, it is beneficial to provide means for testing the acoustic absorption properties of materials, wherein such means are convenient to use, inexpensive, and transportable, such that material samples may be tested without requiring the use of an anechoic chamber.